Is there a formal dictionary somewhere? Both the Lernu! and bab.la dictionaries list "gepatro" as a word. So I'm also inclined to not believe them when they say that "varianto" is a noun for "variant."

The top authority is Akademio de Esperanto, which gives official recommendations, but they don't have a dictionary. What comes closest to being an official, authoritative dictionary is Plena Ilustrita Vortaro, known as PIV. PIV is in E-o only.

Another good one in Reta Vortaro, which publishes its dictionary in machine readable form. PoŝReVo (Ios) and PReVo (Android) applications use that data. Reta Vortaro has explanations in E-o and corresponding words in other languages. Available translations vary from word to word, but usually there is an English equivalent always.

What I usually do, is that search in the Lernu's word list (I wouldn't call it dictionary, since there are no explanations) and check with PIV the usage (PIV has good examples).

When it comes to the word gepatro, the original idea was, that ge- can only be used to denote both genders together, e.g. gepatroj = patro kaj patrino kune. So only with a plural.

But the world has changed. For instance, nowadays a school can have gepatra vespero, a gathering of parents at the evening after the school day, and the teacher informs the parents of something. It doesn't matter, which one of the parents attends. Some think, that it is clumsy to write into the invitation in every spot patro aŭ/kaj patrino so they use gepatro instead. Gepatro also covers cases, where a child has several fathers or mothers.

To summarise some hold with the original use, i.e. only plural, while others accept also singular use.

wswartzendruber:Is there a formal dictionary somewhere? Both the Lernu! and bab.la dictionaries list "gepatro" as a word. So I'm also inclined to not believe them when they say that "varianto" is a noun for "variant."

Yes, there is one official dictionary, which is the Akademia Vortaro which you can find on the Akademio de Esperanto's website.

It lists vari' as part of the Oficiala Aldono 8. The words ant and o are both fundamental. Hence, vari'ant'o is indeed an official word and it is indeed a noun, simply because it ends in -o.

The words ge, patr and o are all fundamental. Hence also gepatro is a word. However its fundamental meaning is "a father of both sexes".

About variant:
The English variant and Esperanto varianto mean exactly the samething: something that deviates from some norm or base model.
English: variant < latin varians (variantis) which is the present participle active of vario, which means I change, alter, diversify.
Esperanto: vari'ant'o < a noun which describes something that varies, is variant or is a variant.

Maybe a more precise Esperanto word would be variaĵo, because it refers to a defined, concrete item.

So my reading is, that none of them is applicable to describe the variants of an aircarft. I would call those as versioj or subtipoj.

A version and a variant are two different beasts (at least according to Oxford dictionary). A variant is a form that differs from a norm. A version is a form that differs from a later or earlier form of the same thing.

In the military, you can have different variants. Let's take the base model (norm) Panavia 200 Tornado. There is one variant for Interdiction Strike, called Tornado IDS. There is one for Reconnaissance, called Tornado RECCE. There is one for Electronic Combat Reconnaissance, called Tornado ECR.
Now there might exist still versions of each of these variants. Versions are in the military generally called Mark Some-Number. For example PC-7 MkII (mark two) is a later version of the PC-7.

What is the Finnish word for variant and is it derived from some root meaning "to differ"?
E.g. in German the translation of variant (relating to aircraft) is "Ausführung", so basically not a different variant, but a different realisation (of the same basic idea).

"Variantti" (variant) and "varianssi" (variance) are used in mathematics, statistics and computer science. "Variaatio" (variation) is used in music and statistics. All those are loanwords with little use outside those fields mentioned above.

"Muuttuja" (variable) is the term used in mathematics, statistics and computer science as well as in the everyday speech. The root is the verb "muuttua" (ŝanĝiĝi). There is a loan word based "variaabeli", which is strictly used within mathematics and statistics, but it is hopelessly old fashioned.

If one want to express, that something differs:
• A differs from B: "A eroaa B:stä", where B is in the elative case (n.b. this expression is also used for divorcing, Mary divorces from John : "Mary eroaa Johnista")
• A deviates from B: "A poikkeaa B:stä"
• A is different than B: "A on erilainen kuin B"

Back into the topic… I would choose any of these modelo, subtipo, versio but not any with vari..